课程内容：计算机网路 课程性质：系必修 课程教师：周承复 开课学院：电资 开课系所：资工 考试日期(年月日)：2011.4.20 考试时限(分钟)：180 是否需发放奖励金：是 (如未明确表示，则不予发放) 试题 : 1. (a)(5%) Give an example of a stateless application layer protocol. (b)(10%) Supposw a web server has 800 ongoing TCP connections. How many serveer-side sockets are used? How many server-side port numbers are used? Briefly(two sentences at most each) explain your answer. (c)(10%) Give two examples of application layer protocols and their port numbers (d)(10%) How doew the transport layer manage to perform application multiplexing and de-mutiplexing? (e)(10%) What does it mean for HTTP1.1 to be "persistent"? What is the value of HTTP1.1 being persistent, as opposed to being non-persistent, as in HTTP1.0? (f)(10%) Please explain what DHT(Distributed Hash Table) is, and how it works. In addition, how we can improve the performance of searching in DHT from 0(N) to 0(log N), where N is the total number of peers in DHT systems. (g)(10%) Please explain the mechanisms "tit-for-tat" and "optimistically unchoke" in BitTorrent. 2.(15%) Consider sending a large file of F bits from host A to host B. A and B are connected by path of L links, and the links are uncongested(that is ,no queuing delays). Host A cuts the file into segments of S bits each(we assume here that F is a mulltiple of S) and adds h bits of header to each segments, forming packets of S+h bits. Each link has a transmission rate of R bps. Find the value of S that minimizes the delay of moving the file from host A to host B. Neglect propagation delay. 3.(20%) Question：A reliavle data transfer protocol Considera sender and receiver that are connected by a sender-to-receiver channel that can corrupt and lose packets. The receiver-to-sender channel is perfect(i.e., it will nnot lose or corrupt packets). The delay on each channel is known to always be less than some maximum value, d. Neither channel will reorder packets. [Note：re-read the channel properties just descrived and make sure you understand them!] You are to design a reliable data transfer protocol for this scenario using only those mechanisms(among the four listed above) that are absolutely required. That is, if you use a mechanism that is not required, your answer will not given full creditt, even if the protocol works. Your protocol should be as simple as possible but have the functionality to reliably deliver data under the stated assumptions. Your solution need not be efficient； it must work correctly. 4. (a)(5%) According to "The Design Philosophy of the DARPA Internet Protocols" Which technique for multiplexing ded DARPA choose (circuit-switching of packet-switching of others)? Why? (b)(5%) There are some goals with priority for the internet architecture in hte paper. If we want to apply the goals to the interner today, should we change the priorty of those goals? Explain wht or why not. 5.(10%) Just choose one to answer (1)According to the papaers about cloud computing you have read, please list two disadvantage to be improved of cloud computing. Also give your reason briefly. (2)What is Iaas, SaaS, PaaS 6.(5%) Suppose k packets arrive simultaneously to a link at which no packets are currently being transmitted or queued. Each packet is of length S and the link has transmission rate T. What is the average queuing delay for the k packets? 7.(5%) Suppose within your Web browser you click on a link to obtain a Web page. The IP address for the associated URL is not cached in your local host, so a DNS lookup is necessarry to obtain the IP address. Suppose that k DNS servers are visited before your host receives the IP address from DNS; the successive visits incur an Round-Trip Time of T1, T2, ...., Tk. Further suppose that the Web page associated with the link contains three objects, consisting oof one HTML text objevt and two small images on the same server. Let T0 denote the Round-Trip Time between the local host and the server containing the objects. Assuming zero transmission tme of the objects. Also assuming the connection is non-persistent and non-parallel. How much time elapses from when the client clicks on the link until the client receives all the objects? -- 解方块不需要思考 但是思考会让解方块更有意义 by aegius1r --

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